Bipolar electrode

ABSTRACT

An improved dimensionally-stable bipolar electrode for use in electrochemical applications comprising a central valve metal layer, suitable anodic material on the anode side of the valve metal, a barrier layer of boron on the cathodic side protected by a suitable common metal cathodic coating. Such electrodes function at low hydrogen permeability rates during use in electrolytic processes.

United States Patent Schultz et al.

BIPOLAR ELECTRODE Inventors: Robert F. Schultz, Niagara Falls;

Edward H. Cook, Jr., Lewiston, both of NY.

Assignee: Hooker Chemicals 8L Plastics Corporation, Niagara Falls, N.Y.

Notice: The portion of the term of this patent subsequent to July 30, 1991, has been disclaimed.

Filed: May 25, 1973 Appl. No.: 363,926

US. Cl 204/290 F; 29/195 A; 29/198 Int. Cl B0lk 3/04 Field of Search 204/290 F References Cited UNITED STATES PATENTS 12/1966 Hall et a1 204/290 F 3,441,495 4/1969 Colman 204/290 F 3,711,382 1/1973 Anthony 204/290 F 3,759,813 9/1973 Raetzsch et a1 204/256 3,826,733 7/1974 Schultz et a1 204/290 F Primary Examiner-Qscar R. Vertiz Assistant Examiner-Wayne A. Langel Attorney, Agent, or F irmPeter F. Casella [57] ABSTRACT An improved dimensionally-stable bipolar electrode for use in electrochemical applications comprising a central valve metal layer, suitable anodic material on the anode side of the valve metal, a barrier layer of boron on the cathodic side protected by a suitable common metal cathodic coating. Such electrodes function at low hydrogen permeability rates during use in electrolytic processes.

10 Claims, No Drawings BIPOLAR ELECTRODE This invention relates to electrodes for use in electrolytic cells. More particularly, this invention relates to improved, corrosion-resistant, dimensionally-stable bipolar electrodes for the electrolysis of aqueous solutions of alkali metal chlorides in the production of alkali metal chlorates.

The electrolysis of aqueous solutions of alkali metal chlorides such as sodium chlorides and potassium chloride has been conducted commercially for years on a wide scale.

Graphite electrodes have been employed in the past in various alkaline chloride electrolysis operations; however, there have been certain disadvantages which have arisen as a result of the use of graphite. One of the most serious of the disadvantages is the constant attrition of the graphite during the electrolysis operation. Attrition results in the increase of the clearance or spacing between the cathode and the anode which in turn causes an increase in the cell voltage drop and a resulting decreasing efficiency in cell operation.

Graphite anodes have a limited life, generally being on the order of about 1.0 inches in thickness oninstallation but at the end of -12 months of continuous use may be reduced in thickness to about 0.25 inches, with the attendant loss in power and efficiency. Such losses, including economic losses, have resulted in the proposed use of metallic electrodes and the use of bipolar cells.

Generally in the production of alkali metal chlorate, bipolar electrodes are now preferentially used which, when arranged suitably in an electrolytic cell, in a spaced electrical series, serve to function both as the anode and cathode in the cell. The electrodes are subjected to an electrical potential while immersed in the alkali metal chloride solution and, electrochemically, alkali metal chlorate is produced, either in the cell itself, or outside the cell upon the standing of the solution.

The advantages of the use of bipolar cells and bipolar electrodes include:

a. bipolar cells are relatively simpler and more economical to produce than are monopolar cells;

b. the electrical contact for supplying current to the electrodes in bipolar cells is applied only through the first and last plates while the current supply to the anodes of monopolar cells must be supplied by electrical contact with each individual anode;

c. bipolar cells allow for the use of minimal distances between the electrodes which reduces voltage and allows for a reduction of the volume of electrolyte used.

Platinum group metal-coated electrolytic valve metals such as titanium have been proposed as substitutes for graphite anodes. The metallic electrodes have offered several potential advantages over conventional graphite electrodes, as for example, lower overvoltage, lower erosion rates and the resulting electrolytic production of higher purity products. The economic advantages gained by the use of such electrodes, however, must be sufficiently high to overcome the high cost of these metallicelectrodes A problem existant with bipolar electrodes based on anodic preciousmetal s, such as platinum, is that the titanium or valve metal support is attacked'by hydrogen during the electrolysis on the cathode side, forming hydrides and causing disintegration of the electrode.

The present invention provides a bipolar electrode having excellent electrolytic use charateristics and excellent durability. The electrodes of the present inven tion are composite bipolar electrodes having a layer of valve metal, preferably titanium, an anodic material, preferably a platinum group metal or metal oxide, de-

posited on one side of the valve metal, and a barrier layer of boron on the opposing side of the valve metal,

overlaid with a layer of suitable cathodic material.

The boron layer has a low hydrogen diffusion rate and effectively prevents the migration of the cathodically produced hydrogen from reaching the valve metal surface.

The electrode comprises a central layer or inner layer of a valve metal of which the oxide is chemically resistant under anodic conditions to the electrolyte employed. The expression valve metal" as employed herein is definitive of a metal which can function generally as a cathode in an electrolytic cell, but not generally as an anode, due to the formation, under anodic conditions, of the oxide of the metal, which oxide once developed is highly resistant to the passage therethrough of electrons.

The preferred valve metal is titanium, although tantalum or colombium may also be advantageously employed.

The expression chemically resistant under anodic conditions hereinbefore employed, as applied to the valve metal, indicates athat the oxide is resistant to the corrosive surrounding electrolyte and is not, to an appreciable extent, subject to erosion, deterioration or to electrolyte attack.

One face of the center or inner valve metal layer is adhered to a layer of boron. The valve metal may be adhered to the boron layer by any means readily available in the art, particularly by sputtering the boron onto the valve metal. The thickness of the layers is not critical, it only being necessary that the thickness of the titanium or other valve metal central or inner layer furnish a self-supporting layer, and that the layer of boron be of such thickness and characterisitics as to provide an essentially hydrogen barrier layer. Generally, the valve metal layer is on the order of from about 0.10 inch to about 0.70 or 0.80 inches in thickness, with the layer of the boron being on the order of from about 0.01 inch in thickness to up to about 0.1 inch in thickness, if desired.

At least an operable portion of the opposing face of the central or inner layer has bonded thereto a layer of suitable anodic material chemically resistant under anodic conditions to the electrolyte used. The term suitable anodic material as employed herein refers to a material which is electrically conductive, resistant to oxidation and substantially insoluble in the electrolyte. Platinum is the preferred anodic material; however, it is also possible to utilize ruthenium, palladium, osmium, iridium, oxides of these materials, alloys of two or more of the metals, or suitable mixtures thereof.

The cathodic material, or outer cathode side layer, may be of any suitable, common cathodic material chemically resistant or insoluble in the electrolyte under cathodic conditions. Such materials as steel, copper, chromium, cobalt, nickel, iron or alloys of these metals may be used. The cathodic material may be applied to the boron layer by any suitable means known to the art which is non-destructive to the boron layer, e.g., electroplating, vacuum deposition, metal spraying or the like. The thickness of the cathodic material may vary, but the material is generally utilized as a thin film, less than about 0.01 inch in thickness.

The anodic material, preferably platinum, can be applied to the anodic side of the valve metal as chloroplatinic acid or as a thermally-decomposable organometallic compound such as platinum resinate. In the use of platinum resinate, the adhesion of the platinum is increased by reducing the concentration of the resinate. For example, the metallic resinate may be mixed with an organic solvent or diluent, such as terpenes or aromatics, typically oil of turpentine, xylene or toluene, before being applied to the base member. The electrode is heated to decompose and/or to volatilize the organic matter and other non-metallic components, leaving on the base member a layer of adherent electroconductive platinum. ln producing a metallic anodic coating by such method, care should be taken to aviod oxide formation, for example, by limiting the temperatures of heating or by effecting heating in an oxygenfree atmosphere such as in a vacuum or under a nitrogen or argon blanket.

Heating may be effected in an air atmosphere; however, temperatures above about 600-650C are not reccommended due to the possibility of oxidizing the valve metal.

In the production of an anodic oxide coating, the temperatures and times of heating are selected that will result in the fomation of an oxide, preferably an oxide of a metal of the platinum group of metal, such as ruthenium. The temperature applied may vary depending upon the particular platinum metal used. Typically, the temperature may be in the range of from about 300 to about 600C, preferably from about 450 to about 550C, with such temperatures applied for periods on the order of from about minutes to about 2 hours. The heating of the metal is most advantageously conducted in an atmosphere containing elemental oxygen such as air or other oxygen-inert gas mixtures although an atmosphere of pure oxygen could be used. The platinum group metal oxide formed is either crystalline or amorphous depending upon the temperatures of heating, with the degree of crystallinity increasing as temperatures and duration of heating are increased. Both crystalline, particularly it the crystals are small, and non-crystalline coatings have good electro conductivity. Where the coatings have a low degree of crystallinity, improved adhesion and conductivity are noted.

It is not necessary that the anodic material be applied in such manner as to completely cover the entire surface of the valve metal central or inner plate. However, the total anodic side of the central or inner plate should be coated with the anodic material to the extent that the massed portion of the anodic material function effectively as an anode. It is preferred that the anodic material essentially cover the anodic side of the valve metal plate.

The anodic layer, preferably a platinum group metal or metal oxide, can be deposited to the extent of 0.0001 inch, although the use of lesser or greater thicknesses may be achieved, depending on the methods of deposition, it only being necessary that the anodic material be present on the anodic side of the central or inner plate in an amount sufficient to function effectively as the anode.

The anodic material, as hereinbefore stated, can be deposited on the central or inner plate of valve metal by any suitable method known in the art. The deposition can be effected, for example, by using a bath consisting .of 4.5 grams platinic chloride and 22 ml 37 percent hydrochloric acid dissolved in 2800 ml water. The temperature is generally maintained between about and C and the current intensity is such that essentially no hydrogen is evolved at the valve metal panel. A graphite anode is used in the bath and the valve metal is made cathodic. The panel is agitated or moved during the plating operation, and the current is regulated as to preclude hydrogen involvement at the valve metal, with the anodic platinum being deposited at a thickness of less than about 0.0001 inch. Minor variations may be effected in the deposition of the precious metal and varying thicknesses may be obtained by suitable modifications in the time consumed in the electroplating operation. Also simultaneous deposition may be made of more than one component, as for example, by effecting a coating containing, in addition to platinum, another platinum group metal compound such as ruthenium, such other component being added to the electroplating bath whereby a desired composite coating is obtained by the elctrodeposition.

The electrodes of the present invention, as stated, find particular application in the electrolytic production of alkali metal chlorates. In producing chlorates using the electrodes of the present'invention, the process may be carried out continuously by passing a solution containing alkali metal chloride through the cells at temperatures generally on the order of up to the boiling point of the electrolyte with the effluent liquor cooled or concentrated to promote crystallization of the chlorate produced in the cell. Advantageously, a small amount of chromate may be added to the liquor fed to the cell to promote chlorate formation, in accordance with methods known in the art.

A typical bipolar electrolytic unit which can be used with the novel electrodes of the present invention consists of a housing having spaced-apart end electrodes with the enclosed space defined by the walls and end electrodes divided intermediate at intervals by the bipolar electrodes into substantially isolated unit cells. For each individual electrolysis cell, there is an individual reaction zone and an individual electrolysis zone substantially isolated from the reaction and electrolysis zones of the adjoining cells, the term unit cell referring to one of the chambers or sections into which the apparatus is divided by the bipolar electrodes. Such cell makeup permits of good circulation of the electrolyte between the zones.

A bipolar electrolytic cell utilizing the bipolar electrodes described has essentially minimal or no current leakage and voltages on the order of 3.8 to 4.0 volts can be employed, with a current density of about 4 amp/in What is claimed is:

l. A bipolar electrode consisting of a core of a valve metal, at least a portion of the anodic surface of which is conductively covered by at least one anodic material selected from platinum group metals and platinum group metal oxides, and a barrier layer of boron on the cathodic side of the valve metal core, at least a portion of the exterior surface of the layer of boron being covered by at least one cathodic material selected from iron, copper, cobalt, nickel and alloys of these.

2. A bipolar electrode as defined by claim 1 wherein the valve metal is titanium.

6 3. A bipolar electrode as defined by claim 2 wherein the anodic material is platinum metal. the anodic material is P metal- 8. A bipolar electrode as defined by claim 1 wherein 4. A bipolar electrode as defined by claim 3 wherein the cathodic material is iron.

the cathode material is nickel.

5. A bipolar electrode as defined by.cl aim2 wherein 5 blpqldr electfoqe defined by claim 1 wherein the anodic material is ruthenium oxide. the matenafl 6. A bipolar electrode as defined by claim 5 wherein A blpolar ekctmde as defined y clalm 1 wherein the cathodic mat rial i i the anodic material is ruthenium oxide.

7. A bipolar electrode as defined by claim 1 wherein 

1. A BIPOLAR ELECTRODE CONSISTING OF A CORE OF A VALVE METAL, AT LEAST A PORTION OF THE ANODIC SURFACE OF WHICH IS CONDUCTIVELY COVERED BY AT LEAST ONE ANODIC MATERIAL SELECTED FROM PLATINUM GROUP METALS AND PLATINUM GROUP METAL OXIDES, AND A BARRIER LAYER OF BORON ON THE CATHODIC SIDE OF THE VALVE METAL CORE, AT LEAST A PORTION OF THE EXTERIOR SURFACE OF THE LAYER OF BORON BEING COVERED BY AT LEAST ONE CATHODIC MATERIAL SELECTED FROM IRON, COPPER, COBALT, NICKEL AND ALLOYS OF THESE.
 2. A bipolar electrode as defined by claim 1 wherein the valve metal is titanium.
 3. A bipolar electrode as defined by claim 2 wherein the anodic material is platinum metal.
 4. A bipolar electrode as defined by claim 3 wherein the cathodic material is iron.
 5. A bipolar electrode as defined by claim 2 wherein the anodic material is ruthenium oxide.
 6. A bipolar electrode as defined by claim 5 wherein the cathodic material is iron.
 7. A bipolar electrode as defined by claim 1 wherein the anodic material is platinum metal.
 8. A bipolar electrode as defined by claim 1 wherein the cathode material is nickel.
 9. A bipolar electrode as defined by claim 1 wherein the cathodic material is iron.
 10. A bipolar electrode as defined by claim 1 wherein the anodic material is ruthenium oxide. 